The non-medical use of pharmaceutical opioids is a growing public health problem throughout the United States. The Ohio Substance Abuse Monitoring Network has reported increases in non-prescribed opioid use since 2000 and documented a link between the misuse of these drugs and transition to heroin abuse. The purpose of this 5-year natural history study is to describe trajectories of non-prescribed pharmaceutical opioid use and identify the factors associated with the development of DSM-IV abuse or dependence on pharmaceutical opioids (and transition to heroin use) among people aged 18-21 years. Respondent-driven sampling will be used to recruit 400 young adults (aged 18-21) in the Columbus, Ohio, area who have used illicit pharmaceutical opioids on 6 or more occasions in the previous 3 months. Each participant will be engaged in a structured interview every 6 months for a 3 year period. These interviews will be complemented with ethnographic research. The Specific Aims are to: 1. Describe key dimensions of illicit pharmaceutical opioid use among young adults as well as the characteristics of those who transition to opioid abuse, dependence, and heroin use, using ethnographic methods. 2. Describe the baseline characteristics of 400 young adult illicit pharmaceutical opioid users and conduct cross-sectional analyses focusing on substance abuse practices. 3. Describe and analyze the trajectories of illicit pharmaceutical opioid use and other drugs over 3 years. 4. Describe and analyze the development of pharmaceutical opioid abuse and dependence and identify the factors associated with transition to disorder. The proposed study is significant because illicit use of pharmaceutical opioids is increasing, and the factors associated with the development of abuse and dependence are unknown. The findings from this study will fill this gap. The study is innovative because the complementary use of ethnographic and quantitative methods will provide a holistic understanding of the natural history of non-medical pharmaceutical opioid use. The findings will help inform the development of interventions to address the problems associated with illicit pharmaceutical opioid use, abuse, and dependence. PUBLIC HEALTH RELEVANCE: Misuse of pharmaceutical opioids is an increasing problem in the United States, particularly among young adults. This study will describe patterns of illicit pharmaceutical opioid use and identify factors associated with transition to abuse and dependence over three years. The results will benefit public health by informing potential interventions.